Hydrogen separation from natural gas

A method comprising:

• measuring the concentration of a light gas in a main flow stream;
• calculating a control parameter as a function of the concentration of the light gas in the main flow stream;
• dividing a portion of the main flow stream to produce a feed stream having a feed flow;
• separating the feed stream by selective permeation across a first semi-permeable membrane to produce a first permeate stream enriched in the light gas and a first retentate depleted in the light gas;
• wherein a ratio of the flow rate of the feed stream to the flow rate of the main flow stream is increased or decreased according to the control parameter;
• wherein an area of the first semi-permeable membrane is increased or decreased according to the control parameter;
• and
• wherein the first semi-permeable membrane comprises a plurality of modules and wherein the area of the first semi-permeable membrane is increased by connecting one or more of the plurality of modules to the feed flow and decreased by isolating one or more of the plurality of modules from the feed stream.